Lapping method



Dec. 28, 1954 'r. cs. LEWIS 2,697,900

LAPPING METHOD Filed May 12, 1953 w A IIIIIIIIIIH# INVENTOR THOMAS G. LEWIS BY YWVWW ATTORNEY United States Patent@ LAPPING, METHOD;

Thomas G; Lewis, Wiimington, Deb, assignlll'. togEf. L, du Pont ale Nemours and Company, Wilmingtn,;Del'.,Q

6 Ciaims.

'lhisinvention relates to; an improved methodzforslapei ping metallic articles, andyparticularlyato a. method fort the lapping of articles fabricated from vnornhomo'geneous 3 or 'inclusion-bearing :metal alloys and metals, or metallic structures displaying non-homogeneous:,characteristicsrto.v abrasiomto develop'thereon an exceedingly high' quality. 0 surface finish.

Metal alloys and some, metals,. to varying degreesyarem non-homogeneous to abrasionand therefore it isi,difii culttolap these materials at a substantially uniform-ratenee-a. essary to develop, high quality surface finishesrwitht good dimentional controL; Typical examples; of; such materials are I the high tungsten vanadium and chromium" content alloy steels, wherein relatively rich carbide,concentrations: exist "at the grainboundaries or at other points Thesecarbides are usually much harder than ,the,metal.:of.the grain interiors and, in the case of vanadium carbide at least, may be of a greater hardness than :aluminum oxide abrasive itself. An even more. serious problem from the. standpoint of lapping is that of the impurity inclusions,- whichare found in all commercial grades of metalalloysa; These inclusions may comprise small amounts of silii cates, sulfides or other'substances, or refractoryparticles picked up from the smelting furnace,;whieh may be either;- harderwor softer, than the primarymetal ponstituent and; thus" possess ,a different abrading characteristics When. 40 metallalloysare lappedby the conventional methods'now} knolwn'yt'o the. art, inclusions invariablyt tear; away; from thew-surrounding jinetal, leaving pits; behind which reduce, thefquality of, the finish. Usually thediSlQdEfid inc1usions,-\ joir'ifthe. abrasive and are forced against the work surface one orflmor'e times during later. passes of the lapping-took thus producing scratches which additionally-mar the fin: ish.;' Yet another cause of pitting; ,is belieyed. to ,be-point' welding, which may occur between the-tool andgtheworkn undefthe high ltemperamrve'swhich 'axisikwithinz thelsm'auwoi -10 which contacts the work during lapping Since the area of contact." The breaking; of point welds under the" relative movement of the lap and the work tears small; pieces of metal away, with resultantpitting,and1the-frea metal particles .i'ntermingle with lthe abrasive,and v deface l thegworksurface upon being, carried into ,thegcontact area during subsequent operation UL SL1 Patent ,2,612,736, property of the sameassignee taught aprocess and a lappingndevicelfor the attainment in materials of the type hereinabovementionedjoftlapped surfaces having qualities of therorde'r, of 1- 2 microinches root-mean-square, and the instant. invention constitutes an improvement over, the patented.processaby whichr it is possibleito obtain appreciably higher surface qualities. High quality surface finishes are ,es'sentia l to certain uses of the materials in polymeric material fi'lrncasting'wheels, polishing rolls for coated fabrics and the Alike. Pure metals, such, as electroplated chromium, for example, aregenerally, free of inclusions but {are (apparently 1 still :nonhomogeneous to abrasion, s'ince itris verydifiicultto; sea cure a high quality finishlthereon while.still -retaining g gooddimensional control.

I havenowfound that it is possible to lapmetal alloys. and also vmetals to, finishes having qualities substantially better than 1.0 microinch' with'high dimensionaliaccuracy 1 by for'cing a diamond dust abrasive against the workwith a balsa wood -l'ap' in the presence of a small amountnof wax :while maintaining a predetermined .pressure=of-contactand'a high relative speedi of. ,translation..of:1the-- sup. faces, whereby allv areas are abraded at substantiallyythe. same rate and, inclusions are not from: from the matrix, 30 metal. -A primary'object of this inventiomitherefore;coma i ICC prises providi'ng a method for the lapping o'f a'r-ticles fab'ricatedfromf metal alloys or metals to exceedingl'y high qualityafinishest- Other'objects include the provisioniofl a method of lapping'articles fabricated from metal alloy's ormetals whichuis rapidand economical,- and which is subject to, high: dimensional control, so that theffinis hed articlesconform to veryprecisesizestandards. The-man ner inwhich these and other-objects'of this invent-i025 are, attained will become apparent from the detailed de-- scription of this specification and'the following drawings j in which:

Fig. 1 isa perspective view of and apparatus adapted I to ,lap a work-piece of cylindrical shape according to this invention, the outline of the workin set up" position being shown by broken line representation;

Fig. 2 isa" rear elevational view showing a preferred" dispositionofiap in relationship to a work-piece of cylindrical shape; I

Fig. 3 isa frontelevation of one embodiment of 12p accordingwto this invention; and

Fig, 4 is a sectional view taken'along the'vertical 'axis of thelap shown in Fig. 3.

Lapping according to, this invention may be'conducted by applying the diamond abrasive andthe wax separately to thebalsa-awood lap prior to commencement of the lappingoperation; however, his preferred to make up a wax stick incorporatingthe diamond substantially uniformly distributedtherein and apply both theidiamond and-the; wax to the lap simultaneously for reasons of convenience ;in;the application and economy in-diamond consumption through the elimination of losses which result wheny-th e-free abrasive is dusted on.

The lap of. thisinventi'on may convenientlycomprise a singlepi ece of; balsa Wood cut transverse=the trunk, the endsgof the fiberspresented constituting a more uniform surface .than is obtained when the wood is cut-with the grain Large laps can'be built up by assembling a multi plicity. of pieces cut across the grain into the lap 'configura-' 1 tion, desired-and-holding the pieces rigidly together by a. suitable device such as. that hereinafter described with referencefto Figs .-@3 andd so that a substantially continuous surface, is provided, for forcing the: abrasive against the work; Sincesit is necessary to maintainihigh relativesurface:speeds during lapping, it is preferredto employ laps of annulan shape in the practice of my m'ethocLforw the reason that this -configuration is stable and strong enough "to withstand very j high rotational speedsw InFig's. ,3 audit, thebalsawood-lap 9 comprises a unitary annulus. cut tran verse -a block of wood, so ithat-u the ,ends of the wood,- fibers make up the working facelap is subjected to relatively high stresses duringpuse', it is desirablerto provide support by mounting thelap tightly within the annularrecess ll of the circular metal holderm 12withla lightdrive fit and to cement securely the sur-;

faces ofithe lapin contact with recess -11'to ho1derw12 with a suitable .high strength commercially available-,ce-

ment, The rear face of holder 12 is provided with tappeda holes l3 for,the receptionof bolts-for attachmenbof; I the lapto, the flange of a powered rotating spindlfia, 1 The;

lap'shown'in Figs. 3 and 4 isadapted-to'thelapping of;

cylindrical .work pieces and the working: face 10' is there; fore. v machined to a; plane, smooth surface -throughout; Where it ,is ;desiredto lap plane '5 surfaces, it is preferred to mach nethe-outer. edgeof face; 10' on a bevel ands -maintain; holden 12 during operationat an inclination- 1 such thatithe beveled edge is parallel to the work surface;- the work and tool being indexed laterally relative toonec. another ,to the; extent necessaryztolap-by successive trav erses-the desired surface area,- whereupon substantially "equalaeras jofithealapcontact-the work-as-in the caseof-q cyllgdr cal worle when the -same "loading pressures are" use Lapping; according ,tothis invention can betcarried 'i outwith conventional-machine shop equipment of the ,type I 1 shown in Fig. :1, wherein the cylindrical work-piece 19.,"3 in broken line representation, is set 'up. betweenithe cenaters andgturned-by a' standard-grinding machine 20; such; as -a-I rown&1Sharpe, Model No. 4, Universal Grinding'w' MElGhll'ljC. The lap, oft-he designt-shown in -Figsv 3 and- 1s P w dy y u yi ot r zed pin l 1, zsuehw" as a Pope Machinery Corporation Series P-2500 design,

which is substituted for the conventional grinding wheel of the grinding machine but is made to traverse the lap lengthwise of the work-piece 19 by the operation of grinding machine 20 in the usual manner. The in-feed of the lap against the work is conveniently gauged by maintaining the horsepower input to the spindle motor at a sufficient level for each of the sizes of abrasive used to maintain high quality lapping at a satisfactory rate. A power demand meter, not shown, of the type represented by the Weston Model 639 Industrial Analyzer is connected in circuit with the motor of spindle 21 to provide the operator with an indication of the existing loading during operation. It will be understood that the particular apparatus hereinabove described constitutes but one arrangement of many which can be employed for the purpose, and that numerous other arrangements known to those skilled in the art may be used, depending on the size and shape of the work to be lapped and the practicable speeds at which the work or the lap can be turned.

In the lapping of cylindrical work, good results are obtained when the surface of the lap is disposed substantially tangent to the surface of the work, as indicated in Figs. 1 and 2. The center line of the lap-powering spindle may be coplanar with the longitudinal axis of the work or may be disposed thereabove; however, it is preferred to locate the lap as shown in Fig. 2, wherein the angle between the line of contact of the lap with the work and the vertical is approximately 45 degs. With this disposition, the right-hand area of contact of the lap passes the work at an inclination of 45 degs. to the right while the left-hand area of contact passes the work at an inclination of 45 degs. to the left, thus providing a broken pattern which is desirable in lapping regardless of the particular method involved. Equally good results are obtained when the lap is turned in either a clockwise or counter-clockwise direction as viewed in Fig. 2.

It is essential to the purposes of this invention to employ a wax in conjunction with the diamond dust abrasive and balsa wood lap. The exact functioning of the balsa wood and wax during the lapping operation is not understood with certainty; however it appears that the peculiar nature of the balsa wood and of the thin layer of wax interposed between the lap face and the work combine to afford an optimum compromise between cutting rate and differential heat generation to achieve the very high surface qualities which are obtained. It is believed that the balsa wood has a softness such that it will not hold the diamond dust against the work with sufiicient force to cause scratches, while still possessing enough retentivity for the diamond to effect a certain amount of lapping. I have found that there is a limiting softness for balsa wood which renders it useless for the purposes contemplated, which can be determined by testing the wood in a conventional Rockwell hardness tester with a 4;" diameter tool steel ball under a loading of 60 kgs. The test is conducted on the wood in an end grain direction and measurement of the diameter of the crater left by the steel ball on cessation of penetration, which usually occurs within about secs., affords a convenient index of the hardness. Indentations of diameters in excess of about are an indication that the wood is too soft for use as a lap. Insofar as my experiments have revealed there is apparently no upper limit on the hardness of the balsa which affects lapping action.

The wax is believed to exist as a highly viscous suspension medium for the diamond under the localized temperature developed during the lapping operatlon, thereby restricting the movement of the diamond particles to the precise extent necessary to effect lapping when backed up by the balsa wood block. ThlS theory is offered as the most logical explanation for the phenomenon observed known to me; however, lapping action is very complex in its nature and perhaps other equally logical theories can be postulated. By differential heat generation I mean the heat generated within local areas being lapped under the action of the lapping tool, such heat when excessive causing unequal expansion of the workpiece which finally results In leccentrlc rotation 1of Stlgc;

r whi in suflicient y serious 0 cause os d ifri fis onal cdii tro l. I have found that the hard waxes, such as beeswax and carnauba, are part1cularly effective in reducing differential heat generation, particularly as regards work-pieces having length/diameter ratios below about 5.0. Where the config of the -P 1S such that this general limit is exceeded, whipping evidencing aggravated differential heat generation occurs in spite I lower than the solid waxes.

of the wax and, once this stage is reached, the operator has no option but must discontinue lapping until the work is cooled, after which operations can be continued.

Liquid waxes appear to be of some utility in eliminating heating but their efiiciency appears to be considerably While beeswax and carnauba wax individually appear to reduce differential heat generation, I have found that a blend of these two waxes in the proportions of about 40-60% carnauba wax to 60- 40% beeswax is particularly effective and I prefer to use this mixture for the preparation of my wax sticks. The sticks are prepared by melting the wax and stirring in the particular grit size of diamond dust abrasive which it is desired to employ until a uniform distribution of the diamond is obtained, whereupon the wax-diamond mixture is poured into cylindrical molds and chilled to solidity before diamond stratification can occur. A typical stick may comprise about 10 gms. of carnauba wax, about 10 gms. of beeswax and 10 carats of diamond dust, the lapping hereinafter reported being conducted with three separate sticks made up with 3200, 6400 and 13,500 grit size diamond dust, respectively. The wax is preferably used sparingly for the reason that an excess contributes to heat generation, it being only necessary to maintain a very thin film over the surface of the work, which can readily be determined visually by the dull luster imparted when sufficient wax is applied. In a typical case about one gram of wax-diamond mixture of the composition hereinabove described, made up with 6400 grit size diamond, was sufficient for the lapping of a 218 sq. in. surface cylindrical metal work-piece in the course of 20 passes of the lap past the work.

In this instance the work-piece was fabricated from Stellite No. 1 of the analysis hereinafter detailed and was cylindrical in form with a diameter of about 6 and a length of about 12". The lap comprised a 1" thick segmental balsa wood ring of 6 /3" inside diameter and 8%" outside diameter, thus providing a working face of 1 width. The work-piece was turned in the grinding machine at a speed of 40 R. P. M., and the lap was turned at a speed such that the relative velocity between the work and the lap was about 8000 ft./min. A power input of 3.0 H. P. was preserved during the operation, and the lap was traversed across the work at a rate of 1" lateral travel per revolution of the work-piece, a single pass requiring about 18 secs. After 10 passes, or 3.0 mins., lapping was discontinued and the work was allowed to air cool for an interval of about 20 mins. before making the final 10 passes. The quality of the work to begin with was about 1.0 microinch R. M. 8., while the final quality was an estimated 0.6-0.7 microinch. It will be understood that the precise ratings of finishes of such high quality as those with which this invention are concerned are approximate, due to the fact that techniques have not yet been developed which permit absolutely accurate measurements within this range; however, all finish ratings herein reported were obtained with a highly sensitive multiple beam interferometer, this instrument being considered the most accurate available.

Observations indicate that effective lapping occurs at work-piece temperatures ranging from room temperature to about 400 F. or even higher, so long as differential heat generation does not cause whipping detrimental to the requirements of dimensional control. The thin wax film on the work blackens somewhat during operation and this visually perceptible color change affords a convenient indication that lapping action is being obtained. In general, I have found that it is desirable with most work to maintain an upper limit of surface temperature of between about -140" F. where high dimensional accuracy is of critical importance. The pressure loading which it is desirable to preserve is not particularly critical, a loading in approximately direct proportion to the diameter of cylindrical work-pieces giving good results for the same materials of fabrication, all other conditions remaining the same. In the practice of my method it is necessary to maintain a relative speed of translation between the contacting surfaces of the lap and the work of the order of about 5000 ft./min., and preferably 7500-10000 ft./min. At speeds in the lower end of the range, i. e., around 5000 ft./min., some materials display the defect known in the lapping art as orange peel, which is effectively prevented if higher relative speeds are preserved. At the other end trs he aug egeo ppin urs t. pe dst nq qand 52b than-210 00 50 0 f /m n-r h wever; he t. g ne.ra.-.. 0 1.. ag av ted at e ey1 .h h speedswithe,

out accompanying advantages and also; the,.lo,w .inherei 1t.

6 sizes pf, diamond, dust together wi h olive .oil. imtheprden; recitedunti a final finis'h of; l-. 2'rnic i'ch.. EMLS quality fwas attainedf The shelhwast balsa" wood lap ."and diamondw'ax app atio' st riength iof the ,balsa woodlap makes it :inadyisable to r. separate steps intheffollowirig chropbl'ogigal' pe e.-. th sextrem -a gotzqfi-P- ci sp rve shout, n

. Q-: f .PP i. Q from their-originaLstate to the final operation, under" the lastistepprne To. finaLtimsh-is aprogressiveoperation, involvingthe useof facilitate themain't'enance of temperature. 0 tr ol mg; a ,se es of,diamond dususizes ranging; from-relatively lapping, the'inand'rel supporting thdshell' wasidrilliil. at; coarse gap-the begin ning ,through ,successively finer sizes l0 two points adjacent the ends of I the,shell l and prfov'ided. to; e nd The;fac es,of the.balsa wooddaps employed with rotary joints for the'introductidn" ridremtiv dp ng each, lapping step,becomes charged locally with cooling water, whichflwas continuously. rrciil e'd hr 'of dramonddust,anditwill;be,understood that a lap which, the interior,oftheishelldi ring thec'o' plte Iap'p'in ath w-p ev sly usedwitha coarser size of,,diamond The surface temperaturevof.thelsliellwas'maii tai "j l-ISI qlll ot,beemployedio attain a higherquality. tweenthe. limits ofl'20fll60l-" Fl 'for 1al{l threela with a finer size,-unless, the ,charged regionjs first removed. by suitably regulating, the flow. of cooling, "ater and a freshlgsur-face:- eXpOSdr;to the:Worke This. can be the shell. In allcases diamond-loade stick accomplished,conveniently. byc'uttingcway thez face of upinthe proportions of 10 gms. car ba hl l1;. l; :'dPl Qlltlr /a' in alathe with a diarn'ond beeswax and1l0carat's .diamonddus't'o each f th point tool- In; addition, balsa wood laps wear unevenly grit ,sizeswere employedfor'the fap'plication o in the course of use, which is evidenced-by a periodic to the balsa woodlap, The shelhwafs ,turried ata fsp bumping? sound-during operation, which becomes more of 55 RIP. M.', and the lap was traversed axiallyat'la rate nfiticeable. as high: spots develop. It is .therefoliepreferred, of 1'; lateral travel .perrevolution. A relativeispeedlofi todress the lap to restore an even contacting surface bework, to lappf about 8000 fee tjper'fminn te was ewe fore burnping, becomes serious enough to, causegblemishes during operation, the lap being'tu'rn'edllat a spee .o'f'lab in the work, and this can be accomplished-by,trimming 3625 R/P. M." i i i the face rof the lap to .a ,depth; of 0.005 -'.'-.0I.-0;1'0 with a, St 1 diamond point tool at interval.ofaboutone-houris.opera; ep tienl. pon completion of lapping. witha given size of Lappingwas initiatedwith .a 3200 ,grit sizestick, the. abrasive,- theeurface, of the workgis cleaned by flushingior- H abrasive being" appliedtothfe, lapiby several s'hortjlight gently;washingwith, a, solvent, such as; carbon tetrapressurestrok'e's.by the. operator and. thelapjbejingftfavr; chloride,followedby a ,finalwashwith watencontaining] erse'd' lengthwise ofthe shell .forone, qnmorejprelim' adfiifitgent Soft absorbent paper or, cotton arepr'efer-v passes to. determine visuallyjifa ,continupus 'laye'i' v H ablymsed to dry thelapped surface and to remove any rewar was laid down. For theusize .of, weir sp.e 1fie,d a mainingtraces:of;abrasive, afterlwhichlapping is started 2 35. application fofabout 1.0 gm, -ofwax diamond.w a s'ufii Wlthjzthfl llfi) finer size. of,abrasive. cient forf, good results, untilfredres'sing becarn ne ssary'} Lapping according to this invention hasproved particu: when, of course, it wasnecessary to add n v'v w 1am (I larly effective for the high quality finishing of the relato ,thelap'to. replacethat', whichwasjr'emo d. The tively hardaalloysteels, of which the following .are represtep .ofwax diamond lapping required 4,0; '3. of lappir rg;- sentative, and .the .loadings for these materials, typified by 40 time, the wheel being redressed three timesat aboptfegual the 440-A example hereinafter reported in' detail, constiintervals, during this period whereupon afinalfsurf ej tute a. guide which, supplemented.by -observations in the. quality. of ,abou't 0l7 to 1.0 rnicroinch j. M. S. was'oli fl course .of .lappingother. materials, willrenablerothersto tained. Thelapfwastlienthere I 1y. ashed ith carbo practice my method: tetrachloride. and Y the outer face as cut. avg y a;

Analysis, in percent Surface, Material 0 Mn P s Si Cr Ni Cu Fe Armco Iron R1104 0.06 0.55 0.013 0.012 0.39 16. 34 4.00 3.90 Balance 39 AISI 440'. 0.60 1.0 1.0 15.0 1.25 Balance 0 Mn P S Si Or Mo V Fe Latrobe Electric Steel Co.s

BR-4 2.4 0.40 0.40 12.8 1.1 4.0 Balance Cr W Balance Stellite No. 1 28-34 11-15 00 55 Ni 00 Cr Mo B Mn 0 All Others X-Alloy 306 40 30 15 8 2.5 1.0 0.5-1.0 2. 5-3.0 5s

A 4" dia. centrifugally cast shell of 12" length and /2" diamond point tool to a depth of about to provide a wall thickness fabricated from commercial quality 440-A fresh surface for Step 2. steel of the anlysis tabulated, having a measured Rockwell St 2 C hardness of 59, was mounted on a mandrel and lapped ep according to this invention in the following manner, using This step was conducted with a 6400 grit size stick in the apparatus shown in Figs. 14. The lap comprised a essentially the same manner as Step 1, except that the 1 thick segmental balsa wood ring of 6% inside diamduration of operation was 1.0 hr. during which it was eter and 8%" outside diameter, thus providing a working found that no redressing of the lap was necessary. The face of 1" width. final surface obtained was of the order of 0.40.7 micro- The shell was first reduced to a diameter 0002-0003 inch R. M. S. The lap was cleaned and refaced as deabove the desired finished diameter, using conventional scribed for Step 1 prior to commencing Step 3. powered grinding equipment. The finish after grinding St 3 was about 16 microinches R. M. S. The shell was then ep lapped with a lignum vitae wood lap in accordance with This step was conducted with a 13,500 grit size stick the method of U. S. P. 2,612,736 in three separate steps of 35 in essentially the same manner as for Steps 1 and 2, ex-

1 hour duration each, employing 800, 1600 and 3200 grit cept that the duration was limited to 1.0 hr. and during the last 10 passes of the lap past the work the loading was reduced to about 1.0 H. P., which seemed to yield a. somewhat improved surface over that secured by maintaining the 2.0 H. P. loading to the very end of operation, as was determined by comparison with the results achieved in an independent test on the same composition material where the latter higher pressure loading was employed throughout. A final surface quality of the order of 0.1 to 0.3 microinch R. M. S. was obtained at the end of this step with retention of good dimensional control.

'From the foregoing it will be understood that my invention may be modified in numerous ways obvious to those skilled in the art without departing from the essential spirit, wherefor it is intended to be limited only by the scope of the following claims.

What is claimed is:

1. A method for the lapping of an article fabricated from a metal alloy or metal comprising forcing a diamond dust abrasive against said article in the presence of a wax with a balsa wood lap, said lap having a hardness greater than that evidenced by a diameter of indentation measuring in excess of about %2" for a /s" diam. tool steel ball pressed against said balsa wood with a loading of 60 kgs. in an end grain direction, while maintaining a relative speed of translation between the surfaces of said lap and said article above about 5000 ft./min. and a loading pressure sufiicient to blacken said wax in the area of contact of said lap with said article.

2. A method for the lapping of an article fabricated from a metal alloy or metal in two separate contactings of the lap with said article wherein the direction of translation of the lap with respect to the surface of said article during one of said contactings is maintained substantially normal to the direction of translation of the lap with respect to the surface of said article during the other of said contactings, each of said contactings comprising forcing a diamond dust abrasive against said article in the presence of a wax with a balsa wood lap, said lap having a hardness greater than that evidenced by a diameter of indentation measuring in excess of about for a- $43" diam. tool steel ball pressed against said balsa wood with a loading of 60 kgs. in an end grain direction, while maintaining a relative speed of translation between the surfaces of said lap and said article above about 5000 ft./min. and a loading pressure sufilcient to blacken said wax in the area of contact of said lap with said article.

3. A method for the lapping of an article fabricated from a metal alloy or metal comprising forcing a diamond dust abrasive distributed in a wax against said article with a balsa wood lap, said lap having a hardness greater than that evidenced by a diameter of indentation measuring in excess of about for a diam. tool steel ball pressed against said balsa wood with a loading of kgs. in an end grain direction, while maintaining a relative speed of translation between the surfaces of said lap and said article above about 5000 ft./min. and a loading pressure sufiicient to blacken said wax in the area of contact of said lap with said article.

4. A method according to claim 3 wherein said wax comprises a blend of carnauba wax with beeswax.

5. A method according to claim 3 wherein said wax comprises a blend of about 40-60% carnauba wax to 60-40% beeswax.

6. A method for the lapping of an article fabricated from a metal alloy or metal in a succession of steps employing progressively finer graded particle sizes of diamond dust abrasive comprising forcing each of said sizes in turn against said article in the presence of a Wax with a fresh balsa wood lap, said lap having a hardness greater than that evidenced by a diameter of indentation measuring in excess of about for a diam. tool steel ball pressed against said balsa wood with a loading of 60 kgs. in an end grain direction, While maintaining a relative speed of translation between the surfaces of said lap and said article above about 5000 ft./min. and a loading pressure suflicient to blacken said Wax in the area of contact of said lap with said article, and cleaning the surface of said article after each said step before beginning the next succeeding step.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,612,736 Lewis Oct. 7, 1952 

1. A METHOD FOR THE LAPPING OF AN ARTICLE FABRICATED FROM A METAL ALLOY OR METAL COMPRISING FORCING A DIAMOND DUST ABRASIVE AGAINST SAID ARTICLE IN THE PRESENCE OF A WAX WITH A BALSA WOOD LAP, SAID LAP HAVING A HARDNESS GREATER THAN THAT EVIDENCED BY A DIAMETER OF INDENTATION MEASURING IN EXCESS OF ABOUT 9/32" FOR A 5/8" DIAM. TOOL STEEL BALL PRESSED AGAINST SAID BALSA WOOD WITH A LOADING OF 60 KGS. IN AN END GRAIN DIRECTION, WHILE MAINTAINING A RELATIVE SPEED OF TRANSLATION BETWEEN THE SURFACES OF SAID LAP AND SAID ARTICLE ABOVE ABOUT 5000 FT./MIN. AND A LOADING PRESSURE SUFFICIENT TO BLACKEN SAID WAX IN THE AREA OF CONTACT OF SAID LAP WITH SAID ARTICLE. 